Mass spectrometric measurement of hydrogen isotope fractionation for the reactions of chloromethane with OH and Cl

Abstract

Abstract. Chloromethane (CH3Cl) is an important provider of chlorine to the stratosphere but detailed knowledge of its budget is missing. Stable isotope analysis is a potentially powerful tool to constrain CH3Cl flux estimates. The largest degree of isotope fractionation is expected to occur for deuterium in CH3Cl in the hydrogen abstraction reactions with its main sink reactant tropospheric OH and its minor sink reactant Cl atoms. We determined the isotope fractionation by stable hydrogen isotope analysis of the fraction of CH3Cl remaining after reaction with hydroxyl and chlorine radicals in a 3.5 m3 Teflon smog chamber at 293 ± 1 K. We measured the stable hydrogen isotope values of the unreacted CH3Cl using compound-specific thermal conversion isotope ratio mass spectrometry. The isotope fractionations of CH3Cl for the reactions with hydroxyl and chlorine radicals were found to be -264±45 and -280±11 ‰, respectively. For comparison, we performed similar experiments using methane (CH4) as the target compound with OH and obtained a fractionation constant of -205±6 ‰ which is in good agreement with values previously reported. The observed large kinetic isotope effects are helpful when employing isotopic analyses of CH3Cl in the atmosphere to improve our knowledge of its atmospheric budget.

Evidence for a major missing source in the global chloromethane budget from stable carbon isotopes

“…The smog chamber set-up and the experimental conditions are the same as recently described in Keppler et al (2018). The samples for the carbon isotope analysis were taken from the same experiments described therein.…”

Isotopic Characterization (2H, 13C, 37Cl, 81Br) of the Abiotic Sinks of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies.

“…Degradation of CH3X in the troposphere is mainly caused by OH and Cl radical reactions (Carpenter et al, 2014) which cause a C-H bond breakage. Consequently, reported hydrogen and carbon isotope effects are large (Gola et al, 2010;Keppler et al, 2018). Halogens are not involved in radical reactions and should 5 only show small secondary isotope effects (Melander and Saunders, 1980).…”

“…For methyl chloride and methyl bromide no multi-element isotopic studies are available yet. Apart from carbon isotope analysis, hydrogen isotope 10 measurements have been demonstrated for CH3Cl (Nadalig et al, 2013;Keppler et al, 2018;Jaeger et al, 2018). For CH3Br bromine isotope analysis was presented in two studies (Horst et al, 2013;Horst et al, 2014).…”

Isotopic Characterization (2H, 13C, 37Cl, 81Br) of the Abiotic Sinks of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies.

“…Degradation of CH3X in the troposphere is mainly caused by OH and Cl radical reactions (Carpenter et al, 2014) which cause a C-H bond breakage. Consequently, reported hydrogen and carbon isotope effects are large (Gola et al, 2010;Keppler et al, 2018). Halogens are not involved in radical reactions and should only show small secondary isotope effects (Melander and Saunders, 1980).…”

“…For methyl chloride and methyl bromide no multi-element isotopic studies are available yet. Apart from carbon isotope analysis, hydrogen isotope measurements have been demonstrated for CH3Cl (Nadalig et al, 2013;Keppler et al, 2018;Jaeger et al, 2018). For CH3Br bromine isotope analysis was presented in two studies (Horst et al, 2013;Horst et al, 2014).…”